In the photograph Thomas Hoepker took on 11 September 2001, a group of New Yorkers sit chatting in the sun in a park in Brooklyn. Behind them, across brilliant blue water, in an azure sky, a terrible cloud of smoke and dust rises above lower Manhattan from the place where two towers were struck by hijacked airliners this same morning and have collapsed, killing, by fire, smoke, falling or jumping or crushing and tearing and fragmentation in the buildings' final fall, nearly 3,000 people